1. Field of the Invention
The invention relates generally to nonaqueous cells and particularly to nonaqueous secondary cells having positive electrodes containing layered chalcogenides as the active material.
2. Description of the Prior Art
There has been considerable interest in recent years in nonaqueous cells because of their potentially high energy densities. Particularly attractive are nonaqueous cells using negative electrodes made with Group I elements, such as lithium or sodium, because the high standard potential and low weight density of these elements afford exceptional possibilities for high cell voltage and high energy capacity per unit weight and per unit volume. Cells having these properties would be useful in any situation in which cell weight and/or volume are critical factors. The positive electrode material should be electrically conductive, because at high discharge rates the energy density depends on the conductivity of the positive electrode material, and also should have properties that enable it to react readily and reversibly with the negative electrode material to enhance secondary battery characteristics. To retain the weight advantages afforded by the negative electrode material, the positive electrode material should also be light.
Positive electrode materials presently contemplated by persons investigating nonaqueous cells include the layered dichalcogenides of the transition metals of Groups IVB and VB of the periodic table. These materials have attracted much interest because of their ability to intercalate a number of species, including lithium, between the layers. The term intercalate is used to mean movement both into and out of the layered structure.
One such layered chalcogenide that appears promising and has been the object of several studies is TiS.sub.2. The TiS.sub.2 structure consists of a sandwich formed by a layer of Ti atoms surrounded on either side by a layer of chalcogens. The negative electrode is made from a species, e.g., lithium, which intercalates between the TiS.sub.2 layers as the cell charges and discharges. Studies performed with techniques such as nuclear magnetic resonance and x-ray diffraction indicate that Li.sub.x TiS.sub.2, for all values of x between 0 and 1, i.e., as the cell goes through a complete charge or discharge cycle, forms a single non-stoichiometric phase. Li.sub.x TiS.sub.2 cells have a middischarge, i.e., x=0.5, voltage of 2.2 volts and an energy density of 480 watthour/kg and are easily reversible for a large number of cycles.
Although seemingly possessing properties making them attractive for use in nonaqueous cells, some layered chalcogenides have not yet been successfully so used. An example is VS.sub.2 which is theoretically more attractive than TiS.sub.2 in a cell using a lithium negative electrode because the values for both the voltage and energy density should exceed the values for TiS.sub.2. The properties of LiVS.sub.2 cells have, however, never been previously reported apparently because both the lack of a method for preparing stoichiometric VS.sub.2 has precluded preparing the cells in the charged state, and more fundamentally, as is now known from the present study, LiVS.sub.2 cells have limited reversibility, approximately 50 percent of the theoretical capacity based on one lithium atom per vanadium atom, at room temperature when put through complete charge-discharge cycles. Although the reason for this limited reversibility at room temperature is not known with certainty, it is believed due to phase changes in the LiVS.sub.2 system as lithium intercalates during a charge and discharge cycle. Li.sub.x VS.sub.2 for x=0 and for x&gt;0.6 has a regular hexagonal structure. The system has slightly distorted monoclinic structures for 0.2&lt;x&lt;0.33 and 0.5&lt;x&lt;0.6. For 0.33&lt;x&lt;0.5 and 0.0&lt;x&lt;0.2, the system consists of two phases.
The presence, at room temperatures, of the additional phases decreases cell capacity to approximately 50 percent of the theoretical value when the cell is cycled at moderate current densities because the intercalation process is not readily reversed, as it is for TiS.sub.2, due to slow attainment of equilibrium conditions. The reason for slow attainment of equilibrium is not known with certainty but appears related to either reduced lithium mobility or a slow rate of phase nucleation. Similar considerations have limited both use and investigation of LiCrS.sub.2 cells.